2mix ACA Booster / Summing Amp suggestions or concepts

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Jan 2, 2021
Hi all,

Been brainstorming some ideas and looking for some alternative fun approaches to this. Excuse the massive thread, just wanted to be as detailed as possible.

So a bit of background:

In the B Room at my studio, we have Pro Tools going out to a modular Tonelux V-Rack with 16 MX2 balanced input modules and then the stereo out from that back into Pro Tools for printing mixes, so essentially a pretty nice sounding 16 channel line mixer.

Since setting up the B Room, we have 16 channels of balanced outputs from RME and Ferrofish units feeding the single V-Rack's sixteen MX2 balanced input modules. The summed stereo signal on the V-Rack terminates to a "Bus Link" DB25 connector on the back - I'm using a hacked-together DB25-to-XLR cable to give me the 2-mix balanced output from the mixer. The output from the Tonelux then needs make-up gain to boost it up to a usable level as the V-Rack output is meant to be summed into another Tonelux summing amp module.

Typically, you'd have the Tonelux all hooked up with some sort of dedicated modular center section -- Tonelux made a discrete summing amp module ("SM2", here is a poor quality diagram of it) with an optional insert on the 2mix. It's great -- we have a full Tonelux console configuration of modular V-Racks (five separate racks for input modules, EQ, faders, preamps+comps, plus a dedicated center/control room section with the "SM2" module) so our B Room studio is like having a little "clone" in order to comfortably bounce projects back and forth between rooms without needing to change Pro Tools routing.

In a pinch, I have this stereo XLR output going into two stereo-linked mic pres on the front of my RME UFX+ converters. Those pres are very clean with low noise floor and sound fine, but I'm looking for something a bit more "exciting" to use it with, along with with a 2mix bus insert option!

My first concept was to build a CAPI 2 Channel ACA & Booster (links to the schematic and block diagram) for this purpose and just have it in a set-and-forget enclosure with a patch point for a 2mix insert. There is a helpful thread here on the forum about them by Jeff. I don't even think I'd need to build out the whole board for what I'm trying to accomplish. I would likely just use the 325 card part of the PCB as the output from the Tonelux is already balanced?

Second concept was to build a stereo Neve line amp - the BA283 AM output stage comes to mind. This person's scheme seems like a simple approach, disregarding the 283 pre sections as they aren't needed for my application.

I know there is also the Majestic Buzz - I could lay out a simplified PCB for this scheme pretty quick. The advantage of this would be that I could add two more L+R inputs in case wanted to add another mixer for more channels in addition to the Tonelux I already have.

I was also looking into the old Telefunken stereo active faders, I think they are models # W690, W691, Neumann 444, something like that, or other Danner Cassettes that'll do the same thing and rack them up. Almost bought an untested one on a whim but then decided against it because I just wasn't sure if it would be the right fit for what I'm looking to do.

For an active stereo booster from my console back into my converters, I'm leaning towards the CAPI option but first wanted to ask the group if there are any other nice D.I.Y. options, or if anyone on the forum has a design of their own. Tube, other flavors, anything!

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